M. S. Zakharov, O. S. Stepanova, V. I. Aparina

Study of Intermetallic Compounds of Gallium with Copper and Gold with Cadmium by Amalgam Accumulation Polarography

(Presented by the scientific seminar of the Department of Physical Chemistry)

For the proper conduct of processes in amalgam metallurgy and for the correct determination of the concentration of microimpurities in high-purity materials by the method of amalgam accumulation polarography, it is necessary to know the behavior of metals in amalgams. In a multicomponent amalgam, metals do not remain indifferent to one another, but may form intermetallic compounds both with mercury and with one another.

According to literature data, gallium forms an intermetallic compound with copper [1, 2], and cadmium with gold [3, 4]. In [2], it was found by an electrochemical method that the intermetallic compound of gallium with copper in mercury has the composition GaCu₂. According to [4], cadmium with gold in mercury gives an intermetallic compound of composition CdAu. It is of interest to study the intermetallic compounds of the indicated elements by the method of amalgam accumulation polarography and to confirm their composition. This method has advantages over the previously used methods for studying intermetallic compounds in mercury—potentiometric and classical amalgam polarography—since it does not require the separate preparation of amalgams and their transfer from one vessel to another, as a result of which oxidation of the amalgam may occur.

The procedures for determining the composition of intermetallic compounds by the method of amalgam accumulation polarography are set forth in [5, 6]. The present work is devoted to the study, by the indicated method, of the intermetallic compound of gallium with copper and of gold with cadmium.

Experimental Part

The investigations were carried out on a type 7-77-4B polarograph with an electrolyzer, the design of which is described in [7]. In studying the interaction of gallium and copper in mercury, a solution of the following composition was used as the electrolyte: 0.1 M KCl + 0.1 M NaSal; in studying the interaction of gold with cadmium, a 0.1 M (NH₄)₂SO₄ solution was used. The content of heavy-metal impurities in these solutions was less than \(1 \cdot 10^{-8}\) M. Two series of experiments were carried out: in the first series, the concentrations of gallium and cadmium were \(1 \cdot 10^{-4}\) M; in the second series, gallium was \(1.6 \cdot 10^{-4}\) M, and cadmium \(3 \cdot 10^{-4}\) M.

Solutions of copper and gold were added to the electrolyzer during the investigation. To obtain reproducible peaks, the solution was vigorously stirred with a magnetic stirrer during electrolysis.

To remove oxygen, the solution was thoroughly purged with nitrogen purified of traces of oxygen. The electrolysis time in all experiments was 6 min. The electrolysis potential in the accumulation of gallium and copper was −1.6 V (sat. cal. el.), and in the accumulation of cadmium and gold, −1.4 V (sat. cal. el.). At these potentials the diffusion current was attained for all the indicated elements. The volume of the solution was 5 ml.

The results of the investigations are presented in Tables 1 and 2.

Table 1

Results of the investigation of the interaction of gold with cadmium in mercury.
Cadmium concentration \(1 \cdot 10^{-4}\) M.

Cadmium concentration \(3 \cdot 10^{-4}\) M.

Table 2

Results of the investigation of the interaction of gallium with copper in mercury.
Gallium concentration \(1 \cdot 10^{-4}\) M.

Gallium concentration \(1,6 \cdot 10^{-4}\) M.

Discussion of Results

The composition of the intermetallic compounds was determined by the methods described in works [5, 6].

Figure 1 shows the dependence of the depth of the anodic cadmium peak on the ratio \(\dfrac{[\mathrm{Au}]}{[\mathrm{Cd}]}\). From the figure it is evident that at this value of the

ratio equal to unity, the anodic peak of gallium disappears. This is due to the formation between gold and cadmium of an intermetallic compound of composition CdAu.

The composition of the intermetallic compound is determined by the method proposed in [6] according to the formula

\[ \frac{n+1}{n}=\lg \frac{X''m}{X'm}\bigg/\lg \frac{*C_1'}{*C_1''}, \tag{1} \]

where \(n\) is the number of atoms of the added element (or a number that is a multiple of the number of atoms) entering into the composition of the intermetallic compound;

\(*C_1'\), \(*C_1''\) are the concentrations of ions in the solution of the element whose concentration remains constant during the investigation (in our case Cd), respectively for the series with the larger and the smaller concentrations of the element.

The physical meaning of \(\lg X''m\) and \(\lg X'm\) is clear from Fig. 2.

To determine the value

\[ \lg \frac{X''m}{X'm} \]

a graph is constructed for two series of experiments in the coordinates \(\lg x-\lg y\)

\[ y=\frac{I}{\,{}^{0}I_1}; \qquad x=\frac{*C_2}{*C_1}, \tag{2} \]

where \({}^{0}I_1\) and \(I\) are the depth of the anodic peak of the element whose concentration in the solution remains constant, respectively without addition of the second element to the electrolyzer and with additions.

Fig. 1. Graph of the dependence of the depth of the anodic peak of Cd on additions of gold. Experimental conditions: 1) \(C_{\mathrm{Cd}}=3\cdot10^{-4}\) M; 2) \(C_{\mathrm{Cd}}=1\cdot10^{-4}\) M; \(V=5\) ml, \(\tau=6\) min; \(E=-1.4\) V (sat. cal. el.); \(r=0.4\) cm.

\(*C_2\) is the concentration of ions in the solution whose content changes (in our case, the concentration of gold).

The values \({}^{0}I_1\), \(I\), \(\frac{*C_2}{*C_1}\) are given in Table 1. For studies of the interaction of gold with cadmium in mercury, the graph in the coordinates \(\lg x-\lg y\) is presented in Fig. 2.

At \(\lg y_m=0\), the values \(\lg x_m''=-0.63\) (at \(*C_1''=1\cdot10^{-4}\) M) and \(\lg x_m'=-1.95\) (at \(*C_1'=3\cdot10^{-4}\) M) are found from the graph. From these data, according to equation (1), \(n:n=0.98\) is calculated. Consequently, the intermetallic compound between gold and cadmium has the composition CdAu, which is in agreement with the data given above and with the data of [4]. These metals form intermetallic compounds of the same composition in alloys without mercury [8].

Figure 3 presents the dependence of the depth of the anodic gallium peak on the value of the ratio \(\frac{[\mathrm{Cu}]}{[\mathrm{Ga}]}\). It can be seen from it that, when the value of the ratio \(\frac{[\mathrm{Cu}]}{[\mathrm{Ga}]}\) is equal to unity, the anodic gallium peak disappears.

Fig. 2. Plot of the dependence of \(\lg y\) on \(\lg x\):
1) for \(C_{\mathrm{Cd}} = 1 \cdot 10^{-4}\ \mathrm{M}\);
2) for \(C_{\mathrm{Cd}} = 3 \cdot 10^{-4}\ \mathrm{M}\).

Fig. 3. Plot of the dependence of the depth of the anodic Ga peak on additions. Experimental conditions: 1) \(C_{\mathrm{Ga}} = 1 \cdot 10^{-4}\ \mathrm{M}\); 2) \(C_{\mathrm{Ga}} = 1.6 \cdot 10^{-4}\ \mathrm{M}\); \(V = 5\ \mathrm{ml}\); \(\tau = 6\ \mathrm{min}\); \(\varphi = -1.6\ \mathrm{V}\) (sat. cal. el.); \(r = 0.04\ \mathrm{cm}\).

Thus, gallium with copper gives in mercury an intermetallic compound of composition \(\mathrm{GaCu}\).

In the general case, the plot of the dependence of the depth of the anodic gallium peak on the ratio \(\frac{[\mathrm{Cu}]}{[\mathrm{Ga}]}\) should have the form of a descending curve.

curve (Fig. 3). This can be explained by a certain degree of dissociation of the intermetallic compound GaCu.

To confirm the conclusion obtained about the composition of the intermetallic compound, we determined the composition of the intermetallic compound between these elements by the method proposed in [6]. To determine the values of \(\lg \frac{X''_m}{X'_m}\), a plot was constructed in the coordinates \(\lg x\)—\(\lg y\), Fig. 4. The data for constructing this plot are presented in Table 2.

Fig. 4. Plot of the dependence of \(\lg y\) on \(\lg x\):
1) \(C_{\mathrm{Ga}} = 1 \cdot 10^{-4}\ \mathrm{M}\);
2) \(C_{\mathrm{Ga}} = 1.6 \cdot 10^{-4}\ \mathrm{M}\).

At \(\lg y_m = 0\), the following values are found from the plots:

\[ \lg x''_m = \overline{2.57}\quad \left(\text{at } C'_1 = 1 \cdot 10^{-4}\right) \]

and

\[ \lg x'_m = \overline{2.2}\quad \left(\text{at } C'_1 = 1.6 \cdot 10^{-4}\ \mathrm{M}\right). \]

The value of \(n\), calculated from equation (1), is equal to 1. It follows from this that gallium with copper in mercury gives an intermetallic compound of composition GaCu. This is in agreement with the literature data obtained by other methods [9].

For the intermetallic compound GaCu, the solubility product was calculated. The latter was found from equation (16) of work [6]:

\[ \lg Lp = n \lg x_m + (n + 1)\lg C_2 - n \lg K', \]

\(\lg x_m\)—is obtained from the plot \(\lg y\)—\(\lg x\);

\(C_2\)—concentration of gallium atoms in the amalgam in the absence of copper; \(C_2\) is calculated from the equation

\[ C_2 = \frac{i_{\mathrm{el}}\tau}{Z F V_k}, \]

where \(i_{\mathrm{el}}\)—electrolysis current of gallium in the absence of copper, A;
\(\tau\)—electrolysis time, sec;
\(Z\)—valence of the gallium ion;

\(F\) — Faraday constant;
\(V_{\mathrm{k}}\) — volume of the mercury drop, \(\mathrm{cm}^{3}\).

\[ K'=\frac{K_{2}}{K_{1}}, \]

where \(K_{2}\) is the electrolysis constant for copper in the absence of gallium;
\(K_{1}\) is the electrolysis constant for gallium in the absence of copper;
\(K_{1}\) and \(K_{2}\) were determined from the following equations:

\[ K_{1}=\frac{i_{\mathrm{el}}}{ZF \cdot C_{1}};\qquad K_{2}=\frac{i_{\mathrm{el}}}{ZF \cdot C_{2}}; \]

\[ \lg Lp=\overline{257}+2\lg 6\cdot 10^{-3}-\lg 18; \]

\[ Lp=2\cdot 10^{-6}\ \text{g-atom}^{2}\,\mathrm{l}^{-2}. \]

Thus, the intermetallic compound GaCu has appreciable solubility in mercury.

REFERENCES

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